Reactive Strength Index (RSI)

Reactive Strength Index (RSI) is a single number — jump height divided by ground contact time — that captures how efficiently you reuse stored elastic energy. It's measured from a drop jump and is one of the cleanest field-applicable tests of fast stretch-shortening cycle performance. Useful for rugby because sprinting, cutting and absorbing collisions all depend on short, sharp ground contacts.

What RSI is

RSI is calculated from a drop jump: step off a box, land, and immediately jump back up as high as you can. The system measures how high you jumped and how long your feet were on the ground. Divide one by the other and you get RSI. Higher is better — you stayed on the ground less time and jumped higher.

Why it matters for rugby

Sprinting ground contact times are short — about 100-150ms at top speed. Cutting and side-stepping happen with similarly brief contacts. Tackle absorption requires reacting against incoming force in milliseconds. All of these depend on fast stretch-shortening cycle performance, which is exactly what RSI measures.

Typical protocol

Drop from a 30cm, 45cm or 60cm box (test multiple heights)
Land and immediately jump back up to maximum height
Minimise ground contact time
Equipment: contact mat, force plates, or OptoJump system
3-5 trials per height, take the best
Find your optimal drop height — where RSI peaks

What good looks like (senior rugby)

2.0-2.8

Backs (good)

RSI from optimal drop height

1.5-2.2

Forwards (good)

Generally lower than backs

>2.5

Elite range

Higher in faster positions